Simmer the kale, in salted water until tender, 7 to 10 minutes. Drain, reserving the cooking water, and chop the leaves. In a large skillet, saute the onion in the oil with the garlic, pepper flakes, and rosemary for about 3 minutes. Add the cooking water to keep the mixture loose. Heat through, taste for salt and season with pepper, and serve with a dusting of Parmesan.